I know many say aún and todavía are interchangeable, but I did read that todavía is "still" and aún is "yet."

Nunca is usually for something you PROBABLY will never do.
For example in many US American schools, kids pledge to never to drugs when they are older-- this might be a case when you use nunca. i.e. "Los niños dicen, "Yo nunca tomo las drogas mientras soy un adulto." However some of these kids may do it when they are older.
Jámas is used for things that will truly NEVER be done.
For example, a vegatarian will never eat meat. i.e. "Los vegatarianos jámas comen carne."

But they use 'yet' at end of a sentence, but 'still' is always before the verb. On the other hand, 'aun' and 'todavia' are used in the same place and you can switch them.

'¿Todavia me amas?' /'¿Aun me amas?'

'Él aún cree en Santa Claus' /' Él todavia cree en Santa Claus'

'Never' and 'Jamas' can be different, maybe, in emphasis. 'Jamas' is a little more emphatic than 'Nunca'. But most of time you can switch both terms with no remorse.

The exeption: When we use 'nunca' with 'mas' an when we use 'nunca ' with 'jamas'. We can say 'nunca mas' and 'nunca jamas' (both sentences mean 'nunca' or 'jamas'), but you can not say 'jamas mas' or 'jamas nunca'.